Referencing is a way to demonstrate that you have extended your reading, learning, and comprehension by using relevant and up-to-date sources.
Primary sources refer to the material the author has written, like all the works written by Shakespeare.
Secondary sources are written as interpretations, criticism, research, etc. about a subject or an author. An interpretation of Shakespeare’s tragedies will be categorised as secondary material on his tragedies. The tragedies themselves will be the primary sources.